Switching power converters powered directly off of an AC power line are commonly designated as off line switchers (OLS). These converters rectify the AC voltage of the line and store it as a unipolar voltage on a voltage storage capacitor. This unipolar voltage, stored on the voltage storage capacitor, is applied to subsequent power switching circuitry for DC to DC power processing.
The input impedance characteristics of an OLS are largely defined by its input rectifier and voltage storage capacitor. The voltage retention and the current sink characteristics of the voltage storage capacitor combine to make the line current conduction time of the rectifiers to be highly peaked and much shorter in duration than the half cycle duration of the input AC voltage waveform. Since the current waveforms are distorted from the desired sinusoidal waveforms the actual current waveforms induce a high harmonic content in the input current. These harmonic signals are returned to the input line and cause EMI problems to the supply network. The power factor at the input is significantly degraded, since the real power is extracted only from the waveform portion of the input current which is in phase with the applied voltage waveform.
Filters are often used at the input of the power converter to block the application of harmonics to the AC line. The filter is normally tuned to eliminate the third and fifth harmonics. Many other harmonicas are generated however and hence, passive filters are limited in their ability to achieve very high power factors. Boost converters have also been used as an active filter input stage to reduce the harmonics at the AC input line where improved performance is required. The boost converter switch is modulated so that the line current waveform is maintained essentially at the fundamental frequency of the applied AC input voltage. A power conversion stage usually follows the boost converter and is needed to provide line isolation and regulation of the DC output signal. Thus, often two cascaded converters are required to achieve low harmonics at the input and achieve input output isolation between the AC input and the DC output.
A particular power converter operative off of an AC power line to achieve high power factors and provide isolation between input and output is disclosed in assignees co-pending patent application Ser. No. 07/590,410 filed Sep. 28, 1991 and entitled "Single Conversion Power Factor Correction Using SEPIC Converter" This converter achieves high power factors, and hence, the current inertia characteristics of its energy storage inductor place large voltage stresses on the converter power switches.